1. Field of the Invention
This invention relates to the testing of underground formations or reservoirs. More particularly, this invention relates to an apparatus and a method for determining properties of formation fluids by detecting and interpreting the refractive index of the formation fluid.
2. Description of the Related Art
Oil and gas companies spend large sums of money in their attempts to find hydrocarbon deposits. They drill exploration wells in their most promising prospects and use these exploration wells not only to determine whether hydrocarbons are present but also to determine the properties of those hydrocarbons, which are present.
For deep offshore fields, before any hydrocarbons can be produced, it is first necessary to spend several years building very expensive platforms with proper oil and gas handling facilities. The design specifications and cost of materials used in these facilities are strongly dependent on the properties of the hydrocarbons, such as gas to oil ratio, viscosity, bubble point pressure, asphaltene precipitation pressure, and so on.
To determine hydrocarbon properties, oil and gas companies often withdraw some hydrocarbons from the exploration well. Wireline formation testers, such as the Baker Atlas Reservoir Characterization Instrument (RCI) can be lowered into the well for this purpose. Initially, fluids that are withdrawn may be highly contaminated by filtrates of the fluids (“muds”) that were used during drilling. To obtain samples that are sufficiently clean (usually <10% contamination) so that the sample will provide meaningful lab data concerning the formation, formation fluids are generally pumped from the wellbore for 30–90 minutes, while clean up is being monitored in real time. Then, these withdrawn fluids can be collected downhole in tanks for subsequent analysis in a laboratory at the surface.
Alternatively, for some properties, samples can be analyzed downhole in real time. Such measurements may be made by sensors in the formation test tool. The formation test tool may be a wireline tool, or a measurement-while-drilling tool. Such sensors may measure properties of the formation fluid, such as for example formation fluid resistivity.
Well logging operations, either by wireline or while drilling, are used to determine properties of the formation fluids, in order to determine the potential hydrocarbon content and the locations of formation water and gas interfaces. These formations may have highly saline brine content. When interpreting deep-reading electric well logs, it is important to know the formation brine resistivity, which is a function of the brine salinity. Sensors used for measuring the fluid resistivity in such tools are commonly limited in their useable sensor range. Some of the highly saline brines encountered have resistivity readings that are beyond the common calibration range of such sensors.
The present invention overcomes these problems by providing an optical apparatus and method for determining brine resistivity from measurement of the brine refractive index.